Rotax ring mount

[Skippydiesel]

sonex892,

"I would in a Sonex and am in the Sonerai, offsetting the engine angles for P factor to the right and up slightly. I have gone with 1.5 deg to the right and 1 deg up. The offset was the actual starting point datum of my sonerai mount design."

I admit this is an area I know little of however it may assist you to know that in my Sonex Legacy, with Rotax 912ULS Mk2 engine bed adapter:

I do not have any P factor in Crusie. I have a modest/expected amount in TO and a bit less in Climb Out.

In straight & level flight, at cruise speed, (130-147 knots indicated, 3000-5500 ft) I did have to carry an unusual amount of up elevator. This was with single 75 kg pilot, full fuel, no weight in the baggage compartment. Using two aproximately 1.5 mm thick "fender washers", I shimmed up the front of the engine (marginally changing the thrust line). My elevator has returned to a near neutral position.

The P factor effect is far more marked for my friend Murray Parr (on this Forum) - he is exploring down a path similar to what you propose.

It seems that yaw caused by engine/prop factors is not the same for all Sonex/Rotax aircraft - the take home message is be careful , you may be correcting something that does not exist in your aircraft, possibly introducing handling characteristics, that will then have to be fixed.

[Skippydiesel]

Working on it. Based on the construction from the guy from Argentina.

Sonex-ring-mount-1.JPG

Sonex-ring-mount-2.JPG

Hi Alex,

I am in awe of those builders who tackle this sort of design/fabrication challenge.

Unfortunatly I doubt my own abilities in this area, so will, whenever possible, be sticking to what I can purchase "off the shelf" (new or secondhand). Where off the shelf is not available, I will with the assistance of skilled service providers come up with something to "do the job".

This brings me to the ring mount; You are making your own - why?
1.Personal challenge?
2. Cost?
3. Unique installation?
4. Combination?

I ask because I am inclined to purchase a second hand Rotax Ring mount and then find some way of connecting it to the Sonex airframe (sufficient challenge for me in itself) BUT if you know of a problem in this plan, please tell me.

[Kai]

Sonex 892 wrote in a posting above:

Quote: If I go down this path with my Sonex I would be modifying a 3300 mount which would also be of no use to you. Unquote.

The reason why the 33A mount is in all likelyhood the best suited for a conversion to a ring mount R912, is of course that of all the mounts Sonex has made available for the Legacy, this is the mount that pulls the engine closest to the firewall. With a set of 12mm flat steel adapter rails to fit a Rotax ringmount R912 it puts he EW CG at 20% MAC, smack on the forward limit on a polished standard gear airframe. I know this works- I am flying around with this mod myself. 120 happy hrs in the air by now without a hitch.

However, I am speculating this does not neccessarily mean that you can´t use anything else. Take for instance the Aerovee: this rather heavy engine has about the same weight as the very light (for the output) Jab 33A . So the Aerovee must also be sitting fairly close to the firewall in a Legacy airframe. With adapter rails along the same line as for the Jab 33A mount, I can presently see only one small, easily correctable issue.

EW CG has of course to be carefully calculated. It will probably move the prop flange of the Rotax somewhat further forward compared to the installed Aerovee, so a cowling mod would probably also be in the works. But it should absolutely be worth closer investigation.

[AlexZ]

You are making your own - why?
1.Personal challenge?
2. Cost?
3. Unique installation?
4. Combination?

1. A little bit. Running a machine/engineering shop for +30 years gives me a bit confidence.
2. Getting a bed mount to Europe is expensive. So yes cost is a factor.
3. No unique installation. I have a complete (spare/for sale) airframe with Aerovee mount hanging in the shop.
4. So the combination is: I have a mock-up to make it all fit and then swap it to the flying one.

I ask because I am inclined to purchase a second hand Rotax Ring mount and then find some way of connecting it to the Sonex airframe (sufficient challenge for me in itself) BUT if you know of a problem in this plan, please tell me.

As Kai wrote the Jab 3300 mount is the best suited to fit a Rotax ring mount. Two flat steel adapter rails and it's an easy fit. If done on the Aerovee mount the Rotax will be 88 mm more forward. Maybe I'll design the rails for it and trial fit a Rotax with ring mount and put it on the scales.

[Skippydiesel]

Good stuff Alex.

My plans built Legacy/Rotax was started by a great builder who unfortunately, for him, had to hand over to me to finish. I don't know what engine mount I have.

What are the distinguishing features of a Jab 3300 mount, compared with an Aerovee ?

[Kai]

Not very much!

It´s easy to distinguish betwen them:

The Jab 33A mount has a sturdy transverse square tube welded in between its two top airframe attach brackets. The Aerovee mount is missing this square tube.

While of the same configuration, there are of course other differences, all aiming for identical position of the engine prop flange and the individual engine attach points. These are more difficuilt to spot.

But you are having a R912 bed mount in your Sonex. Correct? Sonex is the only one supplying these. They can´t be rebuilt to accept the Rotax ring along the effortless lines suggested above.

[Skippydiesel]

Thanks Kai -

There is no square tube on my engine frame.

The Sonex supplied, engine bed : Rotax 912 adapter, does not controle engine movement/vibration, nearly as well as a ring mount.

At some stage in the future, I may be wanting to replace the current Sonex compromise system, with a Rotax Ring Mount but will have to find a way to connect ring mount, via insulators to engine frame. I am hoping for a clever chap to have done this successfully already and be willing to supply detailed photos, measurements, etc.

Note: Rotax specifically advise against using a bed/bottom rail type engine mounting system. This seems to be due to the difficulty such a system has with controlling engine movement and vibration (particularly high frequency vibration).

[sonex892.]

There is no square tube on my engine frame.

Skippy I assume Cliff supplied the plans when you bought the plane? If so I would suggest referring to and studying the pages prefixed "P ".

With the Sonex A models, complete plans were supplied including engine mount drawings, for all aerovee, 2200 and 3300 jabiru. These mounts all differ slightly with the only major common part being the landing gear truss for all 3 and mount weldments for the jabs. The prop flange is in the same place on all 3 of these engines of differing weights. For this reason only, I assume they have placed the prop flange in the same location for the rotax bed mount.

Kai and Alex are referring to using an adaptor fitting to one of these 3 existing sonex designed mounts. Not the bed mount like I think you have.

I agree with Kai's comment regarding considering the altered C of G.

[Kai]

Skippy,

See enclosed a picture of the Sonex manufactured Jab 33A engine mount for the Legacy. Clearly visible is the top square tube connecting the two upper attach brackets to the airframe. The design relieves the two lower engine support lugs with a lot of load.

As far as I know, the Jab 33A mount is the only Sonex manufactured one that incorparates this square tube.

[Skippydiesel]

Thanks Steve/Kai,

As I said, I don't have the the Jab 33A mount, so if I do go ahead with a Rotax Ring Mount, it will have to be mated to the existing Aerovee frame.

Also said, it would be good to see photos/drawings/specifications (measurements) of any successful/proven Rotax Ring Mount, to Aerovee frame (I have no desire to reinvent the wheel).

As for prop flange location - I am unaware of any need/benefit in changing the existing flange location/engine alignment. Should the use of a Rotax Ring Mount (my preferred option) force a change to flange location etc, this may be a good reason not to proceed with the idea.